The invention relates to an optoelectronic semiconductor component having a semiconductor body that is suitable for generating electromagnetic radiation. In the optoelectronic component, an active zone is disposed above a semiconductor substrate, within which zone the electromagnetic radiation is generated in the event of a current flow through the semiconductor body and which zone is disposed between at least one first resonator mirror layer and at least one second resonator mirror layer.
An optoelectronic semiconductor component having a semiconductor body of this type is, for example, a so-called vertical cavity surface emitting laser (VCSEL). In the case of the component, the light generated in the active zone of a heterostructure is reflected perpendicularly to the layer structure having the active zone between the two resonator mirror layers. That is to say in the direction of current flow, and the light is coupled out from the semiconductor body at a steep angle with respect to the surface of the semiconductor heterostructure through one of the reflector mirror layers.
An optoelectronic semiconductor component of this type and its functional principle are disclosed for example in a reference by W. Bludau, titled xe2x80x9cHalbleiter-Optoelektronikxe2x80x9d [Semiconductor Optoelectronics], Hansa-Verlag, Munich, Vienna, 1995, pages 188 and 189, wherein a VCSEL diode is described in which a semiconductor body is applied on an n-conducting substrate. The semiconductor body contains a first layer sequence made up of n-doped mirror layers (lower resonator mirror layer), a region with the active zone and a second layer sequence made up of p-doped mirror layers (upper resonator mirror layer). The electrical connection of the semiconductor body is realized by an ohmic top-side contact on the upper mirror and an underside contact on the substrate. The precise method of operation is described in the above-mentioned literature reference and, therefore, is not explained in any more detail at this point.
The lower resonator mirror layer is, for example, a periodic sequence of alternately GaAs or AlGaAs and AlAs or AlGaAs layers having a high or low refractive index whose respective layer thickness is xc2xc of the wavelength emitted by the active zone divided by the refractive index of the material. The periodic sequence being doped in an n-conducting fashion with silicon and being applied epitaxially prior to the deposition of the active layer sequence on the semiconductor substrate. The reflectivity of the mirror is set by the number of layer pairs. On this n-conducting so-called Bragg reflector, there is applied for example an n-conducting first barrier layer, e.g. composed of AlGaAs, an active zone, e.g. with an InGaAs/GaAs multiple quantum well structure (MQW), and a p-conducting second barrier layer, e.g. composed of AlGaAs, in such a way that the active zone is embedded between the barrier layers.
Adjoining the p-conducting second barrier layer is the upper resonator mirror layer, e.g. a GaAs/AlAs Bragg reflector doped in a p-conducting fashion with beryllium or carbon, on the top side of which reflector is disposed an ohmic top-side contact. After the application of an electric voltage between the top-side and underside contacts, in such a way that the pn junction of the active zone is forward-biased, in the example chosen negative charge carriers are injected from the substrate side through the n-conducting lower Bragg mirror into the active zone. Holes are injected from the top-side contact through the p-conducting upper Bragg reflector.
Similar optoelectronic semiconductor components are described for example in Iga, Inst. Phys Conf. Ser. 145 (8), 1996, pages 967 to 972, and can be produced from different materials for different wavelength ranges of the electromagnetic radiation.
In the case of the VCSEL concept, a large number of lasers can be defined in the lateral direction on a semiconductor substrate and, consequently, it is easy to form laser arrays having more advantageous beam characteristics compared with the so-called separate confinement heterostructure (SCH) lasers.
In the semiconductor laser structures referred to above, the particular problem arises that the p-conducting Bragg reflector made up of GaAs/AlAs, AlGaAs/AlAs or AlGaAs/GaAs layer sequences has a high electrical resistance and therefore causes high electrical losses. Owing to the low thermal conductivity of the above-mentioned materials, the laser diode is consequently heated to a considerable extent during operation. As a result, for example, the lifetime of VCSEL lasers having a high optical output power is severely limited.
Furthermore, the high voltage drops across the p-conducting mirrors prevents the laser diode from being driven with a voltage level of  less than 5 V, which is specified for logic signals.
In order to reduce this problem, the p-conducting mirror layers in VCSEL structures are usually applied on the side of the active zone on which the electromagnetic radiation is coupled out from the semiconductor body. This is because fewer mirror layer pairs are required on this side in order to reduce the reflectivity of this side relative to the opposite n-conducting resonator mirror layer, as a result of which it is possible to couple out the laser radiation. In the case of surface-emitting lasers, therefore, the semiconductor body is usually produced on an n-conducting substrate, as a result of which the p-conducting top side must be given a positive polarity relative to the substrate side. This fact is disadvantageous for the driving of the laser diode, particularly if the targeted, current-regulated driving of a VCSEL diode in a laser array is concerned, as is dealt with in Published, European Patent Application EP 709 939 A1, for example.
Furthermore, it is disadvantageous to produce the GaAs substrates that are usually used in the VCSEL structures described above from p-conducting GaAs, since the latter can be produced with a high structural quality only given a very high technical outlay. They are commercially available, therefore, only with a considerably lower structural quality than e.g. GaAs substrates that are doped in an n-conducting fashion with Si.
Various solution approaches have already been pursued with the purpose of lowering the electrical resistance of the p-conducting Bragg reflectors. In MG Peters et al., J. Vac. Sci. Technol. Volume 12 (6) 1994, pages 3075 to 3083, methods are described in which the transport of holes in p-conducting Bragg mirrors is improved by manipulating the interface material junctions and doping. What is problematic in the case of mirrors based e.g. on InGaAlAs for VCSEL is the large effective mass of the holes and a high energy barrier in the case of the exit of holes, e.g. from a GaAs layer into an AlAs layer. In the case of the methods discussed, the composition of the material is varied in a narrow zone around the GaAs/AlAs interface in different ways between the binary compounds GaAs and AlAs to an AlGaAs alloy and, at the same time, by skillful doping with e.g. Be, C or Si, it is attempted to flatten and minimize the potential barrier.
A further method would be to replace GaAs by the compound AlGaAs in AlGaAs/AlAs Bragg lattices or to replace AlAs by the compound AlGaAs in the GaAs/AlGaAs Bragg lattices. The barrier for holes is thus lowered, as a result of which a smaller electrical resistance is achieved. In this case, however, the fact that the difference in refractive index between AlGaAs and GaAs or AlAs is smaller than in the case of the binary mirrors containing GaAs/AlAs is disadvantageous. It is consequently necessary to apply considerably more mirror pairs in order to obtain a similar reflectivity to that with AlAs/GaAs layer sequences, as a result of which the electrical resistance is again increased.
Furthermore, the thermal conductivity of AlGaAs is considerably lower than that of GaAs or AlAs, as a result of which the thermal energy generated in the laser is dissipated only to an insufficient extent.
A phenomenon that limits the lowering of the electrical resistance in the above-mentioned p-conducting Bragg reflector layers is the occurrence of free charge carrier absorption, which is considerably higher for holes than for electrons. As a result, it is not possible to use acceptor concentrations of arbitrary levels in-the p-type Bragg mirrors. Moreover, if Be is used as acceptor material, the dopant diffuses at customary fabrication temperatures, resulting in a weakening of the desired doping profile at the interface which leads to an increase in the resistance and in the threshold current of the VCSEL.
In the case of other semiconductor materials that are likewise used in VCSEL components, such as e.g. InGaAsP or AlInGaAs or II-VI semiconductors such as ZnMgSSe or BeMgZnSe, similar conditions occur. Added to this is the fact that, for example in the case of VCSEL structures on an InP substrate, the production of p-conducting mirrors is considerably more difficult since the difference in refractive index between the p-conducting mirror pairs used, which are lattice-matched to the InP substrate, such as e.g. p-InP/p-InGaAsP, is very small and it is thus necessary to apply a large number of mirror pairs.
In the case of the production of VCSEL components, in particular of Bragg reflector mirror layers, a high reproducibility with which the layer thicknesses and layer compositions can be set during production using molecular beam epitaxy (MBE) or metal organic chemical vapor phase deposition (MOCVD) forms a basic precondition for consistent component properties. An accuracy of better than 3% should be achieved. Due to complicated variations at the interfaces, in particular in the p-conducting mirrors, this reproducibility can be achieved only with great difficulty.
It is accordingly an object of the invention to provide an optoelectronic semiconductor component which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the electrical resistance of the semiconductor body is reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, an optoelectronic component, including:
a semiconductor substrate; and
at least one semiconductor body suitable for generating electromagnetic radiation disposed above the semiconductor substrate, the at least one semiconductor body, containing:
at least one first mirror layer formed of a semiconductor material of a first conductivity type;
at least one second mirror layer formed of the semiconductor material of the first conductivity type;
at least one active zone in which the electromagnetic radiation is generated in an event of a current flow flowing through the at least one semiconductor body, the at least one active zone disposed between the at least one first mirror layer and the at least one second mirror layer;
at least one first heavily doped, degenerate junction layer of the first conductivity type disposed between the at least one active zone and one of the at least one first mirror layer and the at least one second mirror layer; and
at least one second heavily, degenerate doped junction layer of a second conductivity type disposed between the at least one active zone and the first heavily doped, degenerate junction layer.
Furthermore, the intention is to provide an improved VCSEL component in which electromagnetic radiation is generated in the range between 350 nm and 3 xcexcm, where the electrical resistance of the component is small, the resulting thermal energy is dissipated well and the component is comparatively simple to produce.
The invention provides for the first and the second resonator mirror layer to have a semiconductor material of a first conductivity type, and for one first heavily doped junction layer of the first conductivity type and one second heavily doped junction layer of a second conductivity type to be disposed between the active zone and one of the two resonator mirror layers, in such a way that the second heavily doped, degenerate junction layer lies between the active zone and the first heavily doped, degenerate junction layer. The first and the second heavily doped junction layer preferably have a dopant concentration of  greater than 1*1017 cmxe2x88x923.
Thus, in the case of the optoelectronic semiconductor component according to the invention, the active zone is disposed between the two resonator mirror layers having the same conductivity type. Consequently, only one type of charge carrier is used for electrical transport in the resonator mirror layers. In the heavily doped, degenerate layers, the charge carriers are converted into the complementary type of charge carrier and injected into the pn junction of the active zone. In this case, the sequence of degenerate layers is reverse-biased.
In the case of VCSEL structures based on GaAs semiconductor material, the Bragg reflector mirror layers are preferably constructed to be n-conducting, thereby avoiding the disadvantages described for conventional VCSEL structures, in particular the use of a high-resistance and greatly absorptive p-conducting resonator mirror layer and/or of a p-conducting GaAs substrate that can only be produced with a high outlay.
The sequence of heavily doped layers may be situated on the side of the active zone on which the Bragg reflector having the lower reflectivity is situated. In a different refinement, it may be situated on the side of the active zone on which the Bragg reflector having the higher reflectivity is disposed.
The electromagnetic radiation is coupled out from the semiconductor body either on that side of the semiconductor body that is opposite to the substrate or through the substrate or a hole in the substrate.
Layer pairs containing heavily doped degenerate semiconductor layers of opposite conductor types have already been used to develop optoelectronic components, such as multi-layer radiation detectors, that are described for example in the reference by M. Ilegems et al., titled xe2x80x9cIntegrated Multi-Junction GaAs Photodetector With High Output Voltagexe2x80x9d, in Applied Physics Letters 33 (7) 1978, pages 629 to 631, or multi-layer solar cells, which are described for example in the reference by D. L. Miller et al., Journal of Applied Physics 53 (1) 1982, pages 744 to 748. Furthermore, such pn junctions containing heavily doped degenerate semiconductor layers have been used to monolithically electrically connect stacks of individual semiconductor laser structures in series, as is described for example in the reference by C. P. Lee et al., Applied Physics Letters 30 (10) 1977, pages 535 to 538, or in U.S. Pat. No. 5,212,706.
In the above-mentioned cases, however, the sequence of heavily doped layers is used as an electrical contact between the optoelectronic components for connecting the latter in series. In contrast to this, in the case of the above-described optoelectronic component according to the invention, the sequence of heavily doped semiconductor layers is used to couple a resonator mirror layer of a first conductivity type to a semiconductor layer of a second conductivity type. In this case, parts of the layer sequence e.g. of a VCSEL component, of the kind known from the prior art, for example one or more layers of the resonator mirror layer or a barrier layer are replaced or supplemented by the heavily doped degenerate layers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optoelectronic semiconductor component, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.